Method and apparatus for determining acetabular component positioning

ABSTRACT

An instrument for establishing orientation of a pelvic prosthesis comprises a tripod having an angularly adjustable guide rod on it. The tips of the legs define a plane, and the guide rod is set by the surgeon to a defined orientation with respect to this plane on the basis of preoperative studies. In use, two of the legs of the instrument are positioned by the surgeon at defined anatomical locations on the pelvis (e.g., a point in the region of the posterior/inferior acetabulum and a point on the anterior superior iliac spine). The third leg then lands on the pelvis at a point determined by the position of the first two points, as well as by the separations between the third leg and the other two legs. The position of the guide rod then defines with respect to the actual pelvis the direction for insertion of a prosthesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/134,545, which was filed on Jun. 6, 2008, by Stephen B. Murphyfor a METHOD FOR DETERMINING ACETABULAR COMPONENT POSITIONING and claimsthe benefit of that application as well as U.S. Provisional PatentApplication Ser. No. 60/984,425, which was filed on Nov. 1, 2007, byStephen B. Murphy for a METHOD FOR DETERMINING ACETABULAR COMPONENTPOSITIONING, both of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to determining acetabular component positioning,and provides a method and apparatus for rapidly and accuratelydetermining such positioning.

BACKGROUND INFORMATION

Malpositioning of the acetabular component during hip arthroplasty canlead to dislocation, impingement, wear, and reoperation for all of theseproblems. Proper positioning of the acetabular component during hiparthroplasty requires that the surgeon know the position of thepatient's pelvis during surgery so that the component can be inserted inproper position relative to the pelvis. Unfortunately, the position ofthe patient's pelvis varies widely during surgery and from patient topatient. As a result, large errors in component positioning occur if thesurgeon assumes that the patient's pelvis is positioned squarely withthe operating table, whether in the lateral or supine position. Forexample, one study showed that patients' pelvises were malpositionedthrough a range of 33 degrees around the medial-lateral axis, 47 degreesaround the longitudinal axis, and 17 degrees around theanterior-posterior axis. (Chow J C, Eckman K, Jaramaz B, Murphy S,Evaluation of Intraoperative pelvic position during hip arthroplastyusing computer tomography/radiography matching, International Societyfor Computer Assisted Orthopedic Surgery, 2008.)

To reduce the likelihood of component malpositioning, the position ofthe pelvis can be tracked using computer-assisted surgical navigationtechniques, but the majority of surgeons do not employ these techniques.More basic surgical technique involves qualitative assessment of theposition of the acetabular component compared to the local surroundingbony and soft tissue anatomy that is visible through the incision. Onesuch technique, for example, uses the transverse acetabular ligament asa qualitative marker of orientation of the native acetabulum. (ArchboldH A, Mockford B, Molloy D, McConway J, Ogonda L, Beverland D, TheTransverse acetabular ligament: an aid to orientation of the acetabularcomponent during primary total hip replacement: a preliminary study of1000 cases investigating postoperative stability, J Bone Joint Surg B R.2006 July; 88(7):883-6.)

Unfortunately, such techniques have limited accuracy for severalreasons. First, the transverse acetabular ligament is very close to thecenter of the acetabulum and thus geometrically small errors ininterpretation of its position can lead to large angular errors inorientation of the acetabular component. Second, since the transverseacetabular ligament defines a line, which is a two dimensional geometricelement: it does not provide true three dimensional guidance. Third,since most hips that are worn are also malformed, taking clues from thelocal malformed anatomy is a fundamentally flawed concept. Further, evenif the local anatomy were not abnormal, as may be the case in patientswith femoral head osteonecrosis, it is often the case that the nativeacetabulum is orientated in a much more vertical position than isappropriate for a prosthetic acetabulum. As a result, while the normallyformed native acetabulum may provide clues as to where to position theprosthetic acetabulum, the prosthetic acetabulum should not be placed inthe same orientation as the normal native acetabulum.

SUMMARY OF THE INVENTION

The local acetabular anatomy can be used to determine prostheticacetabular component placement if the relationship of the local anatomyis known relative to overall orientation of the pelvis itself (such asthe anterior pelvic plane) and the spino-pelvic composite. Conversely,if that relationship is not known, the local acetabular anatomy may notreliably be used in determining the position of the prostheticacetabular component. I have concluded that the anatomy of theipsilateral hemipelvis, not the local acetabular anatomy, should be usedto guide the positioning of a prosthetic acetabular component.

Using the ipsilateral hemipelvic anatomy to guide the orientation of theprosthetic acetabular component has several advantages over using localanatomy. First, it is the acetabulum itself, not the hemipelvis, that istypically altered by the arthritic process. Second, parts of thehemipelvis are relatively distant from the center of the acetabulum andso small errors in positioning based on the more distant hemipelvic boneleads to very small errors in orientation of the acetabular component.Third, while there is wide variation in local anatomy due to thearthritic process or due to malformation, there are relatively smallervariations in hemipelvic anatomy, and these variations can be easilystudied by CT imaging or plain radiographs, or can be predictedstatistically.

I have found that there are three points that may be used to dock aninstrument to the ipsilateral hemipelvis to help solve the problem ofacetabular component positioning during hip arthroplasty. These threepoints define an ipsilateral hemipelvic plane. While these may be anythree points on the ipsilateral hemipelvis that are located somedistance apart from one another, an ideal combination of points includesa first point, P₁, in the region of the ischium as it joins theposterior wall of the acetabulum; this point is referred to herein asthe Anchor Point. The ease of identification of this point through mostincisions for hip surgery, its resistance to alteration due to thearthritic process, its position (out of the way of most surgicalmaneuvers performed during reconstructive surgery), and its reliableconsistency of formation, even in hips that have been severely malformedsince birth, make this location an ideal location for an anchor.

A second point, P₂, to be used in forming the Ipsilateral HemipelvicPlane is located on the lateral surface of the iliac wing, immediatelyadjacent to the anterior superior iliac spine. This location is acommonly used location in general, but is unique in its use in thecurrent application. The anterior superior iliac spine is a referencepoint that is frequently used for many things in orthopedics. Inclinical examination, it is frequently used as the proximal landmark tomeasure leg length. In combination with the contralateral anteriorsuperior iliac spine, the position of the pelvis around anantero-posterior axis can be judged so that internal and externalrotation or adduction and abduction of the hip joint can be clinicallymeasured. In surgery performed in the supine position, abduction of anartificial cup can be roughly estimated relative to a horizontal linedrawn between the two anterior superior iliac spines. In image-freecomputer assisted surgery, the anterior pelvic plane can be determinedby digitizing the two anterior superior iliac spines and the pubicsymphysis. In image-based computer assisted surgery, the location of theanterior superior iliac spine can be used as one of many points toperform registration to a CT dataset acquired pre-operatively.

Like other uses of the anterior superior iliac spine, the preferredlocation of P₂ on the surface of the lateral ilium in very closeproximity to the anterior iliac spine takes advantage of the fact thatthis area is readily palpable by the surgeon and can be more preciselypalpated by a sharp instrument that touches the bone. However, unlikeother common uses of the region of the anterior superior iliac spine, inthe current invention, this point is used as one of three points todefine an ipsilateral hemipelvic plane. The distance between P₁ and P₂can be calculated from CT studies, predicted from X-rays, or otherimaging procedures; or even directly measured by the surgeon at the timeof surgery. It can be fixed by the surgeon when he/she selects thisdesired point on the ilium; or it can be selected based on available orfuture statistical studies of pelvic anatomy, considering such factorsas age, weight, sex, race, health and other parameters.

It will be noted that of the three degrees of freedom required to definethe location of P₂ in space, one of them is inherently fixed by the bonesurface itself; one is fixed by selecting the distance d₁₋₂; and one isfixed by the surgeon when he/she places the point at a specific locationon the surface and at a given distance from P₁. The fact that thesurgeon is defining only one of these three parameters necessary toidentify P₂ in space makes this point a very reliable one to identify,and very resistant to variation or significant error.

The third point, P₃, that defines the ipsilateral hemipelvic plane maybe located anywhere on the ipsilateral hemipelvis. For both mathematicalstability and for the physical stability of an instrument designed todock on these points, it is best that P₃ be as far distant from Points 1and 2 as is reasonably practical within the limits of the hemipelvicsurface; in a region where no vital structures will be encountered; on aregion of the bone that is relatively dense, so that an instrumentdocked on this point will typically not inadvertently penetrate throughthe bone; and in a region where the bone surface is at least locallyrelatively flat so that an instrument docked to this point on thesurface will not slip.

These three points, taken together, give very reliable informationregarding the position of the ipsilateral hemipelvis and thus theposition of the overall pelvis, and a mechanical instrument docked tothese three locations therefore provides a guide for the proper relativeorientation of the acetabular component itself during surgery.

Although the location of P₃ may be established in a variety of ways, Ihave found that it may be established very simply by locating it at aprescribed ratio “r” of the distance d₁₋₂ between P1 and P2 such thatthe distance d₁₋₃ between P₁ and P₃ is given by d₁₋₃=r·d₁₋₂, where theoperator “·” denotes multiplication. Similarly, the distance d₂₋₃between P₂ and P₃ is given by d₂₋₃=r′·d₁₋₂, where r′ is likewise aprescribed ratio of the distance d₁₋₂ between P₁ and P₂. r and r′ may bethe same or a different ratio.

I have found that ratios r, r′ of from approximately seventy percent toapproximately one hundred percent of the length d₁₂ nearly alwayspositions P₃ on a flat surface of the ilium, on generally firm bone,proximal and anterior to the sciatic notch. Within this range, I havefound a ratio of approximately ninety percent to be most oftensatisfactory, with both r and r′ approximately the same, i.e.,r=r′=0.90. Although any ratio or combination of ratios between the P₃and Points 1 and 2 may be sufficient, a ratio of approximately thismagnitude reliably places P₃ away from vital structures that might beharmed by a different positioning. Further, providing a sufficientlydistant spacing of the points in this manner provides an intrinsicstability to an instrument that is docked to them. Additionally, of thethree degrees of freedom of P₃, one is prescribed by the distance fromP₁, the second is prescribed by the distance from P₂, and the third isprescribed by the surface of the bone itself. Therefore, the location ofP₃ is entirely determined by the surgical instrument and the bonesurface, and is not subject to variable interpretation by the surgeon.

As discussed above, the three Points define an ipsilateral pelvic plane.The orientation of this plane with respect to a standard reference planesuch as the anterior pelvic plane may readily be determined from CT orother method of 3D imaging, from X-ray imaging, by average, bystatistics, or by other known techniques. Thus, the orientation of aprosthetic component defined in relation to this ipsilateral hemipelvicplane may readily be converted to an orientation relative to anotherplane such as the anterior pelvic plane, a standard reference for pelvicsurgery.

For example, preoperatively, one may create a three-dimensional model ofthe patient's pelvis and hips based on medical imaging such as CT or MRand define the anterior pelvic plane (APP) from this model in the usualmanner (typically, by marking the pubic symphysis or pubic tubercles andright and left anterior superior iliac spines). This is the referenceplane commonly used by surgeons because the landmarks can readily bepalpated so as to correlate the CT image with the patient anatomy. Next,an ipsilateral plane is defined on the 3D model in accordance with theinvention, i.e., a first point in the region of the posterior inferioracetabulum, a second point adjacent to the anterior superior iliacspine, and a third point, preferably on the ilium, and spaced from theother two. As mentioned above, I have found that the third point isadvantageously spaced from the first and second points at a distance ofabout from 0.7 to 1.0 times the distance between the first and secondpoints, preferably at around 0.9 or 0.8. Although the spacing betweenthe first and third points and between the second and third points neednot necessarily be the same, I have found that an approximately equalspacing usually works well.

To assist the surgeon in defining the desired orientation of theprosthetic acetabulum relative to the ipsilateral pelvic plane, I havedevised an instrument that accurately and rapidly facilitates fixing thelocation of P₂ and P₃, once the anchor point, P1, is selected by thesurgeon. The instrument is essentially a tripod, i.e., it has threelegs, one for each Point that is to define the ipsilateral pelvic plane.In use, a first leg is positioned by the surgeon at the anchor point onthe pelvis. The second leg is positioned by the surgeon at the selectedpoint, P₂, on the pelvis. The third leg is adjusted to extend a selecteddistance or distances from both P₁ and P₂ on the pelvis and therebyestablishes the location of P3.

An acetabular direction guide is then placed on the tripod. The guide isadjustable to any desired orientation relative to the tips of the tripodlegs (which coincides with the three Points on the pelvis when theinstrument is rested on the pelvis). The orientation of the acetabulardirection guide is selected by the surgeon as providing the desiredorientation for insertion of a prosthetic component. The guide itselfmay be used to direct the orientation of the acetabular component, orthe guide may be used to direct the insertion of a pin into the pelviswhich will guide the ultimate insertion of the prosthetic component atthe selected orientation.

The tripod may take a number of forms. For example, it may take the formof a camera tripod, in which three legs emanate from a central socketwhich enables adjusting the angles between each pair of legs (and thusthe distance between their tips). Preferably, however, it takes the formof a generally planar frame having two arms extending generally from acommon hub and oriented at an angle to each other. Legs located on thehub and on each arm, respectively, extend outwardly from the planarframe. Advantageously the legs extend perpendicular to the plane of theframe and are preferably of equal length. The tips of the legs contactthe pelvis, and thereby establish the three points that define thepelvic plane.

In one implementation of the invention described herein, the tripodtakes the form of a pair of extensible arms emanating from a central hubaround which the arms pivot, much like a draftsman's compass. The lengthof the arms are adjustable by the surgeon, as is the angular openingbetween them. The three legs extend perpendicularly from the planedefined by the arms, with one leg extending from the central hub and thetwo other legs extending from the ends of the arms.

In another implementation of the invention, the tripod takes the form ofa pair of extensible arms emanating from the central hub at a fixedangle, but is essentially the same as the implementation describedimmediately above.

In still another implementation, the leg that is positioned at P₁ is ahollow leg that slides over a guide pin that is drilled or screwed intothe bone at P1. In use, the surgeon first fixes the guide pin into thebone at the desired location, then slides one of the legs of the tripodinstrument over the guide pin This anchors the instrument. The legs thatland at P₂ and P₃ are preferably pointed to pierce the skin and stayfirmly on the bone surfaces at P₂ and P₃ without slipping.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a schematized view of a left pelvis showing three points fordefining an ipsilateral hemipelvic plane in accordance with theinvention;

FIG. 2 is a view in perspective of a prototype instrument used to testthe concept of the invention;

FIG. 3 is a view in perspective of one embodiment of an instrumentsuitable for use in establishing the reference planes and orientationsin accordance with the invention;

FIG. 3A is a partial view of the instrument of FIG. 3 form the direction3A-#A in FIG. 3;

FIG. 3B is an illustrative sketch showing the fitting of a leg to aconduit; and

FIG. 4 is a view in perspective of yet another embodiment of aninstrument suitable for use in establishing the reference planes andorientations in accordance with the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

FIG. 1 is schematized view of a right pelvis showing the three points inquestion. Distance d₁₋₂ is the distance between the first point, P₁, (apoint located on the ischium as it joins the posterior wall of theacetabulum) and the second point, P₂, a point located on the lateralsurface of the iliac wing, immediately adjacent to the anterior superioriliac spine. Distance d₁₋₃ is the distance between the first point, P₁,and the third point, P₃ (on the ilium); and distance d₂₋₃ is thedistance between the second point, P₂ and third point P₃. The distanced₁₋₂ is the baseline; the distances d₁₋₃ and d₂₋₃ are set at a distanceof from 70% to 100% of the baseline distance, preferably about 80-90%.In FIG. 1 they are shown as being approximately 80-85% of the baselinedistance. In FIG. 1, distances d₁₋₃ and d₁₋₃ are shown as approximatelyof equal length, although, as noted above, they need not necessarily beso. These three points, P₁, P₂, and P₃ define an ipsilateral plane.

The surgeon defines the desired orientation of the prosthetic acetabularcomponents and the alignment indicator with respect to this ipsilateralpelvic plane. This may be done directly with respect to the ipsilateralpelvic plane so defined, or it may be done with reference to theanterior pelvic plane as established, for example, from a 3D computermodel of the patient's pelvis based on CT, MR, or other imaging. Whilepatient specific individual 3D models may be the most accurate method,other methods of determining the desired orientation of the alignmentindicator and the ipsilateral pelvic plane based on statistical averagesof pelvic structures and sizes of patients of similar size, sex, weight,age and/or other known characteristics may be used.

FIG. 2 is a view in perspective of a prototype instrument used to testthe concept of the present invention. The prototype was formed fromreadily-available components in order to quickly test the concept. Theinstrument has first and second arms in the form of metal rods 10, 12,respectively, emanating from a hub 14 comprising first and secondadjustable clamps 14 a and 14 b to hold the arms 10, 12, respectively,in a desired positioned when tightened. Arm 10 was formed from rods 10 aand 10 b secured together by an adjustable clamp 10 c to enableextension of rod 10 b with respect to rod 10 a to a desired position.Similarly, arm 12 was formed from rods 12 a and 12 b secured together byan adjustable clamp 12 c to enable extension of rod 12 b with respect torod 12 a to a desired position.

At the outer extremity of arms 10 b and 12 b are legs in the form ofrods 16 and 18, respectively, held in adjustable clamps 10 d and 12 d,respectively. Similarly, a leg in the form of a rod 20 extends fromclamp 14 a. In the prototype, the clamps were formed from readilyavailable washers and nuts which can be loosened and tightened to hold acomponent in a desired position. The legs 16-20 were standard trocharshaving a pointed tip to enable extension through the skin of a patientand secure lodgment on the pelvic bone. The legs extended downwardly thesame distance from the arms 10 and 12 and the hub 14 and thus the armswere essentially in a plane parallel to the plane defined by the tips ofthe legs.

The legs are shown positioned on a model of the pelvic anatomy forperformance of a surgical procedure on the right acetabulum. The firstleg, 16, rests on a point, P1, the anchor point, located on the ischiumas it joins the posterior wall of the acetabulum. The second leg, 18,rests on a point, P2, located on the lateral surface of the iliac wing,immediately adjacent to the anterior superior iliac spine; the thirdleg, 20, rests on a point, P3, on the ilium; the location of this pointis determined by the length of the arms 10 and 12 as set by the surgeon,as well as the relative angular orientation of these arms and thesurface of the bone.

A direction guide 22 is adjustably secured to the frame. This guide willdefine the orientation for insertion of a prosthesis or the performanceof other surgical procedures. The orientation of the guide with respectto the reference points P₁, P₂, and P₃ is established by the surgeon inpre-operative or intra-operative planning. When placed on a patient inthe manner described above, the instrument defines an ipsilateral pelvicplane with respect to which the orientation of the guide 22 may readilybe established by imaging, by direct measurement, or by other knownmeans. From CT or other imaging studies, the orientation of this planewith respect to the commonly used anterior pelvic plane may bedetermined, and thus the orientation of the guide with respect to theanterior pelvic plane may be established if desired. During subsequentsurgery, when the instrument is positioned on the patient in accordancewith the reference points, the guide provides the surgeon with areference axis for insertion of a prosthesis such as an acetabulum at adesired orientation. In the prototype, the guide was secured to one ofthe arms (e.g., arm 12) by means of an adjustable clamp 24. Although theguide is shown in FIG. 2 as a simple solid rod, it may advantageously bea hollow tube through which a pointed rod may be inserted into thepatient. The rod is screwed into the bone and the instrument thenremoved, leaving the rod as the guide for acetabular component insertionor other procedure. Alternatively, a hollow tube may be mounted parallelto the guide and a pin inserted through this tube into the patient'sbone to set the desired orientation for insertion of a component or forother surgical procedure.

In use, the surgeon places the patient in an appropriate position forthe procedure to be performed. For example, in replacing the acetabulumof the right pelvis of a patient, the patient may be placed on his orher left side and a surgical field of sufficient size is created inorder to expose the right acetabulum. The surgeon selects the point P1,preferably in the region of the ischium as it joins the posterior wallof the acetabulum. This is the Anchor Point, and is generally exposed inthe surgical field during the operation, so that it is readily located.Leg 16 is positioned on this point. The surgeon next selects a point,P2, on the lateral surface of the iliac wing, preferably immediatelyadjacent to the anterior superior iliac spine. This region is usuallyreadily located by palpation, and need not be within the exposedincision, but rather simply within the sterile surgical field. Thesurgeon then adjusts the length of arm 10 (and, if necessary, arm 12) toposition leg 18 on P2, while ensuring that leg 20 lands on thehemipelvis, and preferably on a relatively flat area so that the legwill not slip. This defines point P₃.

Placing the first and second legs on the patient as discussed above, andfixing the distances d₁₋₃ and d₂₋₃ between the legs in the mannerdiscussed above, fixes the point at which the third leg of theinstrument will land on the patient. For acetabular component placement,this point will preferably be on the ilium, generally above the sciaticnotch. The area in which it lands is a relatively strong portion of theanatomy, and thus is amenable to receiving the third leg to provide astable reference plane on the patient. However, it is devoid ofsignificant landmarks, and thus without the aid of the presentinstrument would not serve as an area which would help to establish areference for component placement.

As noted previously, a simple but effective rule for adjusting thelengths of the arms 10, 12 is to set the distance d₁₋₃ between P1 andP3, as well as the distance d₂₋₃ between P2 and P3, at some ratio of thedistance d₁₋₂ (the baseline distance) between P1 and P2. A ratio of fromabout 70% to 100% appears to work satisfactorily, but I have found thata ratio of about 80-90% works well in most cases.

FIG. 3 is a view in perspective of another embodiment of an instrumentin accordance with the invention that is more suited for replication andwidespread use. A turret 50 is pivotally mounted on a base 52 forrotation about a vertical axis 54. (In this connection, it will behelpful to view also FIG. 3A which is a partial view of the instrumentfrom the direction 3A-3A in FIG. 3). Wings 56, 58 on the turret mount aguide 60 for rotation in a vertical plane about a horizontal axis 55that is transverse to axis 54. First and second extensible arms 62, 64are mounted in base 52 for rotation with respect to the base and thus toeach other. This enables the surgeon to adjust the angle α between thearms as desired. Of course, this may also be accomplished with one fixedarm and one rotatable arm. (It will also be understood that both armsmay be fixed at a set angle α.)

Extensions 62 a, 64 a, extend from arms 62, 64, respectively. Theextensions 62 a, 64 a may telescope from their respective arms as shownin FIG. 3, for example, in a manner similar to that of a camera tripod.Alternatively, they may be mounted for sliding overlap with these armsor may use other known forms of extension. Mounted at the ends ofextensions 62 a, 64 a are hollow guides 66, 68, respectively. Legs 70,72, have sharpened tips 70 a, 72 a, respectively, and heads 70 b, 72 b,respectively. The legs preferably also have a short boss 72 c (see FIG.3B) for mating with a corresponding groove on the interior surface ofconduits 66, 68. The legs extend downwardly from the arms and the hub tothe same extent, and thus the arms are essentially in a plane parallelto the ipsilateral plane formed by the tips of the legs.

In use, the surgeon grasps the heads 70 b, 72 b of the legs 70, 72 andinserts them into the conduits 66, 68 so that the bosses align with thecorresponding grooves in the respective conduit. He or she then rotatesthe heads to thereby removably lock the legs into the guides. In similarfashion, a leg 74 having a sharpened tip 74 a and a head 74 b isremovably insertable into, and lockable within, the base 52. Thisconstruction facilitates cleaning of the instrument by enabling therapid disassembly and reassembly of the legs. Of course, the legs couldalso be permanently fixed to the respective arms.

The tips 70 a, 72 a, and 74 a of the legs 70, 72, 74, respectively,define a plane, the ipsilateral plane. Axis 54 is perpendicular to thisplane, while axis 55 is parallel to it. Thus, the orientation of guide60 relative to the ipsilateral plane is defined by the orientation ofthe guide relative to axis 55 (which defines the azimuthal orientationor angle of the pointer) and axis 54 (which defines the elevationalorientation or angle of the pointer). To facilitate setting ordetermining these angles, scales (not shown) may be attached to, ormarked on, the turret 50 and the base 52.

The instrument of FIG. 3 is used in the same manner as that of FIG. 2,i.e., for operation on the right hip, the tip of leg 70 is located onthe selected point P1, the tip of leg 72 is located on selected pointP2, and the lengths of these legs (as well as the angle α ifappropriate) is adjusted to ensure that the tip of leg 74 lands on thehemipelvis, preferably in a relatively flat region, to thereby fix pointP₃. This orientation of the instrument thereby recreates the ipsilateralplane defined in the preoperative studies. When the guide 60 is then setto the angles determined as desirable in the preoperative studies, it isoriented with respect to the patient in the desired manner. It may thenbe used to guide the component insertion tools along the appropriatedirection. To do this, the surgeon may simply visually align the toolwith the guide during the insertion. Preferably, however, the guide 60is hollow and an elongated pin is extended down the guide while it is inthe desired position on the patient, and the pin is then anchored intothe patient. The instrument may then be removed, the pin preserving thedesired orientation for placement of the prosthesis.

The important parameters of the instrument can be easily adjusted sothat the instrument can effectively be customized to each patient in avery short period of time, taking just a minute or two, and can be donewhile the instrument is sterile on the operating table. Importantadjustments are the lengths of arms 62 and 64, the angle between thesearms, and the orientation of the alignment indicator. While the angle(alpha) between arm 1 and arm 2 can be adjustable, the preferred anglecan be fixed at approximately 67.5 degrees, which is the angle definedif the distances between P1 and P3 and between P2 and P3 are both 0.9times the distance between P1 and P2. Similarly, while the length ofarms 60 and 62 need not be the same, the preferred embodiment of theinstrument is such that the arms are of approximately the same lengthrelative to each other, although the length of arm 1 and arm 2 variesfrom one individual to the next.

The patient-specific adjustable variables for each operation then arethe lengths of the arms and the orientation of the alignment indicatoror guide relative to the plane of the instrument body. The lengths ofthe arms can be determined before surgery or even during surgery using anumber of methods, each with a differing degree of precision. The mostprecise method of determining the desired lengths of the arms is bythree dimensional imaging of the individual patient, typically by CT orMR imaging. In this way, points P1 and P2 can be determined by thesurgeon on the computer model and the location of P3 can beautomatically calculated as the unique point lying on the bone surfaceat a specified distance from P1 and P2. Less precise, but potentiallysuitable methods of determining the lengths of the arms include:

-   -   1. An overall average of all patients;    -   2. An overall average of all patients of the same sex, height,        weight, diagnosis;    -   3. As statistically predicted from measurements from        magnification corrected plain radiographs of the patient to be        treated and matching those radiographs statistically with        radiographs and three-dimensional reconstructions of similar        patients who have had CT, MR, or other methods of deriving        three-dimensional models. This method can be referred to as 2D        to 3D statistical modeling; or    -   4. Directly measuring the distance between P1 and P2 at surgery,        which can even be performed using the instrument itself.

Next, the orientation of the alignment indicator relative to the planeof the instrument body must be decided. This determination is based ontwo or more factors. The first is the relative orientation of thehemipelvic plane (P1, P2, P3) relative to the overall pelvis and/or theanterior pelvic plane. The second is the surgeon's desired cup positionrelative to the overall pelvis and/or the anterior pelvic plane in thecase of acetabular cup replacement surgery. Finally, an additionalvariable might include an adjustment for anticipated orientation of theoverall pelvis for various activities and positions after surgery. Thisthird variable is mentioned because there are variations in the waydifferent patients' pelvises are orientated, some with more pelvic tiltand some with less pelvic tilt. (Klingenstein G, Eckman K, Jaramaz B,Murphy S. Pelvic Tilt Before and After Total Hip Arthroplasty,International Society for Computer Assisted Orthopedic Surgery, 2008.)This is particularly true of patients with a fused lumbosacral spine. Ifthe individual postoperative patient pelvic orientation can be predictedpreoperatively, then this factor can be incorporated into the planningof the desired orientation of the alignment indicator.

As with determination of the lengths of the arms, orientation of thealignment indicator can be determined by the above combined withknowledge of the anatomy of the individual patient's pelvis, aided bydetermination of one or more of the factors discussed above. Using thesemethods of adjusting the instrument body, adjusting the alignmentindicator, and applying the instrument in surgery, the desiredorientation of an acetabular component can be rapidly and reliabledetermined during surgery. Further, the method avoids the unreliableinfluences of local anatomy that is frequently deformed and furtherdistorted by the arthritic process.

FIG. 4 shows yet another embodiment of the invention in which theextensible arms are formed by slidable beams and in which the conduitsholdings the legs are themselves removable for ease in cleaning. Inparticular, the instrument has first and second arms 102 and 104 formedfrom first and second arm segments 102 a and 104 a, respectively, fixedto and extending from a hub in the shape of a cylindrical conduit 105having a central bore extending therethrough. Markers 102 c, 104 c onarm segments 102 b, 104 b, respectively, indicate the amount ofextension of the arm segments, and thus the length of the arms. In theconfiguration shown, the arm segments 102 a, 104 a are positioned at afixed angle to each other, advantageously on the order of 67.5 degrees.

A mating rod segment 110 having a leg 116 extending therefrom is snuglybut removably press-fitted into the conduit 105 to form one leg of thetripod 100. Similarly, rod segments 106 and 108, having legs 112, 114,respectively, extending therefrom are snugly but removably press-fitinto hollow-bored conduits 107, 109, respectively, formed on the ends ofarms 102 b, 104 b, respectively. Removal of the legs facilitatessterilization of the instrument before each use, and also makes theinstrument more compact for storage.

A first plate 120 is fixed to the hub 105; the plate has a scale 120 athereon. A second plate 122 is pivotally mounted on the hub 105 forrotation in the horizontal plane about a vertical axis 124 with respectto the first plate 120. A releasable lock 123 fixes the angularorientation of the plate at the orientation set by the surgeon. A guide128 is pivotally mounted on the second plate for rotation in thevertical plane about a horizontal axis 126. Plate 120 has a scale 120 afor indicating the angular orientation of the plate 122 with respect toit (the azimuthal angle). Similarly, plate 122 has a scale 122 a forindicating the angular orientation of the guide 128 with respect to theplate (the elevation angle). As was previously the case, the tips 112 a,114 a, and 116 a of legs 112, 114, 116, respectively, define a plane(the ipsilateral hemipelvic plane) and the arms 102 and 104 are parallelto this plane. Thus, the orientation of the guide can be referred tothis plane and thus also to the anterior pelvic plane if desired.

In this embodiment, the conduits 112, 114, 116 are detachable from thearms 102 and 104 and the hub 105, respectively. Advantageously, they maysimply form a forcefit, although other means of connation may be used.This facilitates sterilizing the instrument for repeated use, whileenabling its ready reassembly in the operating room. It also enables theinstrument to be stored in a more compact package.

In still a further variation of the instrument of FIG. 4, one of thelegs, e.g., the leg 112, rather than being removable, may in factcomprise a hollow conduit of the same length as the legs 114 and 116.With this configuration, the surgeon may first select the anchor pointP₁, insert a pointed rod (trochar) into the anchor point, slide the hub106 onto the rod, and then position the legs accordingly as describedabove, while being sure that the instrument frame will not slide fromthe hip during positioning.

A further variation in the use of the instrument involves itscombination with computer-assisted surgical navigation using optical,electromagnetic, or other means of tracking. In any of these processes,a coordinate system for the pelvis must be defined, where thiscoordinate system be the anterior pelvic plane or another plane. Sincethe mathematical relationship between the ipsilateral hemipelvic plane,as defined by the mecahnical instrument, and any other plane, such asthe anterior pelvic plane, can be determined preoperatively, placementof the instrument onto the ipsilateral hemipelvis and then measurementof its location using a navigation system can be used to rapidlydetermine the orientation of the pelvis. The mechanical instrument canthen be removed and surgical navigation can proceed as usual thereafter.

Similarly, surgical navigation can be facilitated by rapidly determiningthe overall orientation of the pelvis using the same method as above,but with a virtual mechanical instrument. In this way, P1 is determinedby the surgeon using a digitizer (optical, electromagnetic, or other).Next, the surgeon defines P2 using the digitizer. The virtual instrumentdefines the distance P1-P2 and so P2 must lie on a sphere whose radiusis P1-P2 and must also lie on the bone surface. As such, two of threedegrees of freedom for determining the point P2 are predetermined andonly one of the three degrees of freedom for determining the point P2 issubject to surgeon choice. This greatly improves the accuracy ofdetermining this point. Finally, the surgeon defines P3 using adigitizer. Using the virtual instrument, the location of point P3 mustlie on the bone surface, must be a specified distance from point P1, andmust be a specified distance from point P2. Thus, the virtual mechanicalinstrument defines all the three degrees of freedom that determine thelocation of the point P3 and its location is not subject to surgeonchoice. Further, this very specific point can be determined on a flat,indistinct surface where there are no palpable landmarks.

What is claimed is:
 1. A method for orienting a surgical instrument withrespect to the pelvis of a subject, comprising: placing a first leg of atripod in the area of the posterior inferior acetabulum of said subject;placing a second leg of the tripod in the area of the anterior superioriliac spine of said subject; and placing a third leg of the tripod onthe ilium of said subject, the ends of the legs in contact with thesubject thereby defining a plane with respect to which said instrumentmay be oriented.
 2. A method according to claim 1 in which said firstleg is placed proximate to, and outside of, the acetabular rim.
 3. Amethod according to claim 2 in which said legs are separated by defineddistances relative to each other.
 4. A method according to claim 3 inwhich the distance between said first and third legs is adjusted to be adefined proportion of the distance between said first and second legs.5. A method according to claim 4 in which said proportion is determinedin accordance with subject-specific data obtained from a CT scan.
 6. Amethod according to claim 4 in which said proportion is determined inaccordance with subject-specific data obtained from an X-ray.
 7. Amethod according to claim 4 in which said proportion is determined inaccordance with a combination of subject-specific data obtained from anX-ray and statistical data defining distance relationships between theacetabulum and locations on the ischium.
 8. A method for registering asurgical navigation system with respect to a subject, comprising:placing a first leg of a tripod in the area of the posterior inferioracetabulum of a subject; placing a second leg of the tripod in the areaof the anterior superior iliac spine of said subject; and placing athird leg of the tripod on the ilium of said subject, the ends of thelegs in contact with the subject thereby defining a plane with respectto which said navigation system may be oriented.
 9. A method accordingto claim 8 which further includes the step of determining theorientation of said navigation system to said plane in accordance withsubject-specific data obtained from a CT scan.
 10. A method according toclaim 8 which further includes the step of determining the orientationof said navigation system to said plane in accordance withsubject-specific data obtained from an X-ray scan.
 11. A methodaccording to claim 8 which further includes the step of determining theorientation of said navigation system to said plane in accordance with acombination of subject-specific data obtained from an X-ray andstatistical data defining an estimate of the three dimensional surfacestructure of the hemipelvis and pelvis.
 12. A method for registering asurgical navigation system with respect to a subject, comprising:registering a first point in the area of the posterior inferioracetabulum of a subject; registering a second point in the area of theanterior superior iliac spine of said subject; and registering a thirdpoint on the ilium of said subject, said points defining a plane withrespect to which said navigation system may be oriented.
 13. A methodaccording to claim 12 which further includes the step of determining theorientation of said navigation system with respect to said plane inaccordance with subject-specific data obtained from a CT scan.
 14. Amethod according to claim 12 which further includes the step ofdetermining the orientation of said navigation system with respect tosaid plane in accordance with subject-specific data obtained from anX-ray scan.
 15. A method according to claim 12 which further includesthe step of determining the orientation of said navigation system withrespect to said plane in accordance with a combination ofsubject-specific data obtained from an X-ray and statistical datadefining an estimate of the three dimensional surface structure of thehemipelvis and pelvis.
 16. A method for registering a surgicalnavigation system with respect to a subject, comprising a virtualinstrument wherein the first of three or more points is digitized in thearea of the posterior inferior acetabulum of a subject; the second pointis digitized in the area of the anterior superior iliac spine of saidsubject at a predetermined distance from the first point; and the thirdpoint is digitized in a location that is on the bone surface, aspecified distance from the first point, and a specified distance fromthe second point of said subject, these three points digitized on thesubject thereby defining a plane with respect to to which saidnavigation system may be oriented.
 17. A method according to claim 16which further includes the step of determining the orientation of saidnavigation system with respect to said plane in accordance withsubject-specific data obtained from a CT scan.
 18. A method according toclaim 16 which further includes the step of determining the orientationof said navigation system with respect to said plane in accordance withsubject-specific data obtained from an X-ray scan.
 19. A methodaccording to claim 16 which further includes the step of determining theorientation of said navigation system with respect to said plane inaccordance with a combination of subject-specific data obtained from anX-ray and statistical data defining an estimate of the three dimensionalsurface structure of the hemipelvis and pelvis.